1. Field of the Invention
The present invention relates to articles made from elastomeric material, particularly pneumatic tires, reinforced with rubberized fabrics comprising cords with at least one shape memory wire; and also to the said fabrics and to the corresponding cords.
The invention also relates to a process for the manufacture of these rubberized fabrics.
2. Description of the Related Art
Many articles made from elastomeric materials, including neumatic tires for vehicle wheels, conveyor belts, flexible hoses for the transport of fluids and similar, comprise at least one rubberized fabric formed by a plurality of reinforcing cords, normally textile or metal, disposed parallel to each other and incorporated in an elastomeric material.
In the following port of the present description, the wording "elastomeric material" is intended to denote the composition of the incorporating material as a whole, in other words the rubber, including the polymer base, the reinforcing fillers and the various protective, accelerating, anti-ageing and other agents, the whole according to recipes which are well known to those skilled in the art.
It is also known that metal cords are formed from a plurality of single metal wires wound spirally with respect to each other, with predetermined intervals, according to a plurality of configurations which are well known to those skilled in the art.
In general, the cited articles require cords having particular characteristics of mechanical strength when exposed to various stresses, including tensile and compressive stresses, and having corrosion resistance. Corrosion may be initiated in the metal wires of the cord by the presence of moisture in the residual air inside the cords incorporated in the rubber, or by direct contact with water when the breaking of the rubber layer exposes the cord to the external environment.
Once initiated, the corrosion may be propagated along the wires in the absence of a suitable protective coating of the wires.
To meet the requirement of corrosion resistance, it is convenient for the space between the metal wires of the cord to be completely filled with rubber to avoid the presence of air incorporated between the wires and subsequent formation of moisture with consequent development and propagation of the corrosion phenomenon.
Additionally, in order to resist mechanical stresses, the wires of the cords must be closely associated with each other in order to ensure correct behaviour in operation, as represented graphically, in a Cartesian stress-strain diagram, by a substantially linear characteristic.
In fact, due to the distance between the wires, a cord is subject to mechanical hysteresis and to a risk of failure of the wires, even under a compressive load lower than that withstood by a cord in which the distance between the constituent wires is minimal or zero.
The requirements of good penetration of the rubber between the wires and high performance of the cords in operation are particularly important in pneumatic tires; these are normally made by assembling a plurality of different semi-finished components, some of which consist of strips of various sizes formed from the previously cited rubberized fabrics.
The manufacture of the rubberized fabrics for pneumatic tires is carried out by incorporating the bare cords in an elastomeric material, preferably by means of known rubberizing devices, such as extruders and calenders, supplied from feed reels of the bare cords disposed before the said devices. It is during this stage of incorporation that the penetration of the elastomeric material into the cords has to be achieved.
There are various known solutions designed to ensure good penetration of the rubber into the cord, all characterized in that the cords which are easily penetrable by the rubber do not have optimal behaviour in the pneumatic tire during its use.
In one solution suitable for stranded cords, the cord comprises a first pair of wires disposed in one plane and a second pair of wires disposed in a further plane which rotates with respect to the first along the longitudinal development of the cord, so that in each cross section the surfaces of the wires have maximum exposure and consequently maximum coating with elastomeric material. This solution entails a non-uniformity in the disposition of the wires along the development of the cord, with unsatisfactory performance in use.
A different solution specifies cords in which the wires are kept slack (open cords) so that a small distance is left between them. In the passage through the rubberizing device, the distance set between the wires permits good penetration of rubber into the cord. This solution may cause the compacting of the wires against each other, owing to the tension to which they are subjected even before they reach the device, thus making it impossible or very difficult for the rubber to penetrate into the cord; when this does not happen, the cord is rubberized in an optimal way but maintains a behaviour which is hysteretic, and therefore unsatisfactory, in use.
A further solution specifies the disposition in the cord of a wire having a non-linear (zigzag) configuration, so that a space is provided between each of the various wires and the next, and the penetration of rubber to the centre of the cord is promoted. This solution entails lower fatigue resistance of the non-linear wire and therefore of the whole cord.
If we now examine cords of the multilayer type, these comprise a central core covered with a plurality of concentric layers of wires, as in the case of the known cord having a 3+9+15 configuration, in other words a core of three wires twisted together, round which is wound a first layer of nine wires on which is wound a second layer of fifteen wires. These cords are used, in particular, in the casing plies of pneumatic tires for trucks.
In this cord, little rubber penetrates into the inner layer, and practically none penetrates into the core, owing to the physical barrier created by the radially outer layers of wires. In these types of cord, in order to achieve sufficient rubber penetration, the solution based on the use of wires of different diameters is convenient.
Although on the one hand this solution improves the rubber penetration, on the other hand it is unsatisfactory in respect of the performance of the cord in use.
To improve the characteristics of the behaviour of the pneumatic tire in use, metal cords in which at least one of the component wires is made from an alloy of a shape memory material have recently been used.
Shape memory materials are described, for example, in pages 3 to 20 of the publication "Engineering Aspects of shape memory alloys", Butterworth-Heinemann, published in 1990.
Shape memory wire, as will be described in greater detail subsequently, has the properties (1) of possessing a precise memorized shape which is imparted to it by a heat treatment carried out at a specified temperature which imparts to the wire a predetermined critical point, (2) of losing this shape as a result of mechanical stresses imparted at a temperature below the critical point, and (3) of returning to the memorized shape whenever its temperature exceeds the critical point.
For use in pneumatic tires, this type of wire, which has been heat treated so that it has, for example, an undulating shape, is subjected to a stretch which imparts another configuration, for example linear, at ambient temperature, before it is stranded with the other wires to form a cord.
Whenever the temperature in the pneumatic tire increases, for example as a result of high speed, to a point higher than the critical point of the shape memory wire, the wire tends to return to the originally memorized undulating shape.
However, since the shape memory wire is stranded with the other wires and the whole cord is fixed to the elastomeric matrix, and the whole structure is subject to tension, this wire is unable to contract to assume its own undulating configuration of lesser length.
Consequently, there is an increase in tensile stress in the shape memory wire (the wire acts as a stretched spring), the effect of which is to increase the rigidity of the structure in opposition to the effect of centrifugal force.
In particular, U.S. Pat. No. 5,242,002 describes a radial tire whose belt assembly comprises three belts, the first two having cords symmetrically inclined with respect to the equatorial plane and the third having cords disposed circumferentially.
The cords are formed from a plurality of wires wound spirally with respect to each other and each cord of the inner belts comprises a plurality of metal wires, at least one of which is made from an alloy of a shape memory material.
Japanese patent application JP 4362401 relates to a radial tire having a belt structure whose outer layer comprises a two-way shape memory expansion element, preferably an element of the spring type made from a Ni--Ti alloy, wound in the circumferential direction (at 0.degree.) on the underlying belt layers.
The shape memory element tends to contract in the circumferential direction when the tire is subjected to heating in high speed travel. However, since this contraction is impeded by the underlying belt structure, the element develops a tensile force which makes the belt assembly more rigid, thus controlling the phenomenon of expansion of the tire at high speed.
At low speeds or in normal conditions of use, the shape memory element maintains the initial shape or returns to the initial shape as a result of the inflation pressure. The applicant has perceived that the failure to achieve optimal behaviour as described above may depend on the particular behaviour of the said cords with shape memory wires which, together with their advantages, pose a considerable problem.
What happens in practice is that, during the vulcanization of the tire, which, as is well known, is carried out at a temperature of the order of 150.degree. C. and sometimes above, in its initial stage, when the rubber compound has low viscosity, the contraction of the shape memory wire causes the opening of the cord, in other words the spacing apart of the component wires.
The rubber is then vulcanized, losing its plasticity, but the cord is unable to close up, being prevented from doing so by the contraction of the shape memory wire, and is therefore consolidated in the vulcanized tire in this swollen configuration, with all the cited disadvantages of unstable behaviour and low compressive strength, resulting particularly in poor resistance to the bending and compression stresses.
The cited patents U.S. Pat. No. 5,242,002 and JP 4362401 fail to deal with this aspect, and therefore the problem of improving the penetration of the elastomeric material between the wires of a cord while obtaining good performance of the cord, and consequently of the tire in use, remains substantially unresolved at the present time.